Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint=true, openWorldHint=false and destructiveHint=false, so safety is covered. The description adds real context beyond that: it discloses that verification status is included in the results and clarifies a genuinely confusing capability boundary (Send never receives mail; use check_mailbox for Correio receiving).

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Purpose is front-loaded in the first sentence, and the follow-on sentences each add distinct value. The third sentence is somewhat dense and parenthetical, but it is not wasted text.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a parameterless read-only list tool with no output schema, the description covers what is returned (domains with verification status) and prevents a foreseeable misuse (expecting receiving info from Send). Little else an agent needs is missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Zero parameters with 100% schema coverage, so the baseline is 4. There is nothing parameter-related the description could add.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ('List sending domains') and adds the key returned attribute, verification status. It implicitly separates itself from get_domain/create_domain by the list verb, but never names a sibling explicitly.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Usage is implied rather than stated: 'Emails can only be sent from a verified domain' explains why verification matters, not when to call this tool. The third sentence usefully routes the agent to check_mailbox for the receiving side, which is a partial alternative pointer but not a when/when-not rule.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.